Undesirably, NOx can be produced in a variety of processes including combustion processes. For example, NOx is produced by internal combustion engines (whether mobile or stationary), gas turbines and coal- or oil-fired power plants, refining processes, by refinery heaters and boilers, furnaces, by processes of the chemical processing industry, by coke ovens, municipal waste plants and incinerators, coffee roasting plants etc.
One method of treating NOx in an exhaust gas of an internal combustion engine is to absorb the NOx from a lean gas in a basic material and then to desorb the NOx from the basic material and reduce it using a stoichiometric or rich gas. Such a method is disclosed in EP 0560991. In EP 0560991, a NOx absorbent comprises alumina supporting a basic material such as an alkali metal, alkaline earth metal or a rare earth metal and a precious metal, such as platinum. The mechanism described in EP 0560991 for NOx absorption from an oxygen-rich gas is that oxygen (O2) is deposited on the surface of the platinum in the form of O2− and nitric oxide (NO) in the gas reacts with the O2− on the surface of the platinum and becomes NO2 (2NO+O2→2NO2). According to the mechanism described, subsequently, a portion of the NO2 is oxidized on the platinum and is absorbed into the absorbent, e.g. barium oxide (BaO). While bonding with the BaO, the NOx is diffused in the absorbent in the form of nitric acid ions NO3−. The description explains that whilst the mechanism is explained by using platinum and barium loaded on the carrier, “a similar mechanism is obtained even if another precious metal, alkali metal, alkali earth metal, or rare earth metal is used”. This combination of a basic material for such as an alkali metal, alkaline earth metal or a rare earth metal and a precious metal, such as platinum and possibly also a reduction catalyst component such as rhodium is typically referred to as a NOx trap or a NOx absorber catalyst (NAC).
WO 2004/030798 discloses an exhaust system for a diesel engine comprising a NOx absorbent component, which is devoid of platinum, followed by a NOx-trap comprising at least one NOx absorbent and platinum. The platinum-free NOx absorbent component can be selected from alkaline earth metal (calcium, magnesium, strontium and barium are mentioned) compounds, alkali metal (e.g. potassium and/or caesium) compounds and rare earth metal (such as cerium, yttrium or praseodymium) compounds. The platinum-free NOx absorbent can be supported on a suitable support, such as particulate alumina, silica, zirconia, titania, ceria or a mixture or composite oxide of any two or more thereof. Alternatively, the platinum-free NOx absorbent can comprise the support per se, such as ceria or alumina. In addition to the NOx absorbent component, the platinum-free NOx absorbent can include a base metal catalyst for oxidising nitrogen monoxide to nitrogen dioxide in lean exhaust gas, and a manganese compound, a cobalt compound and a copper compound are specifically mentioned.
WO 2004/079170 discloses an exhaust system for a lean burn internal combustion engine comprising a particulate filter, a first NOx absorbent disposed upstream of the filter and a second NOx absorbent disposed downstream of the filter. The first NOx absorbent can be selected to release stored NOx during lambda>1 conditions at about 300° C. and above and in this regard suitable NOx absorbent components comprise at least one of cerium, lanthanum, alumina, iron, zinc, calcium, sodium, magnesium and mixtures of any two or more thereof.
EP 1054722 discloses a SCR system for treating combustion exhaust gas containing NOx and particulates, comprising in combination and in order an oxidation catalyst effective to convert at least a portion of NO in said NOx to NO2 and enhance the NO2 content of the exhaust gas, a particulate trap, a source of reductant fluid, injection means for such reductant fluid located downstream of said particulate trap and an SCR catalyst.
EP 1559892 discloses an exhaust gas purifying apparatus for an internal combustion engine having an exhaust system. The exhaust gas purifying apparatus includes a catalyst for absorbing NOx in the exhaust gas. When the air-fuel ratio is set to a value on the rich side with respect to the stoichiometric ratio, absorbed NOx is reduced to ammonia and the generated ammonia is retained in the catalyst. The catalyst reduces NOx with the retained ammonia when the air-fuel ratio is returned to a value on a lean side with respect to the stoichiometric ratio. An illustrative catalyst comprises platinum on alumina mixed with ceria (CeO2) or platinum on ceria as a NOx absorbent in a first layer and a transition-metal ion exchanged zeolite as an ammonia absorbing second layer overlying the first layer (see also “A NOx Reduction System Using Ammonia Storage-Selective Catalytic Reduction in Rich and Lean Operations”, N. Satoh et al. presented at the 15th Aachen Colloquium “Automobile and Engine Technology”, 9-11 Oct. 2006).
Mercedes has recently announced the launch of the E320 Bluetech™ model in USA. The Dieselnet website report of the launch (available at http://www.dieselnet.com/news/2006/09daimler.php) includes the following explanation:                “While the marketing name “Bluetec” has been derived from the urea—called AdBlue in Europe—selective catalytic reduction (SCR) technology, two different NOx control options are introduced in the Bluetec line-up: a NOx adsorber catalyst (NAC) and urea-SCR. In the 2007 model year, the E320 emission control system includes a close-coupled diesel oxidation catalyst (DOC), followed by the NAC converter, the diesel particulate filter, and an SCR catalyst. The NOx adsorber stores NOx emitted during lean operation, followed by regeneration at a rich exhaust condition, which is periodically achieved through an engine management strategy. Regeneration in diesel NAC catalysts is typically performed at a frequency on the order of 2 minutes, and lasts a few seconds. During the regeneration, the NAC catalyst produces some ammonia, which is stored in the downstream SCR catalyst, and used to further enhance NOx reduction through the SCR reaction.”        
U.S. Pat. No. 5,656,244 discloses a system for reducing cold start NON, carbon monoxide and hydrocarbon emissions from mobile source exhaust wherein molecules of NOx are adsorbed onto a regenerable sorbent material during the ineffective warm-up period of a three-way catalytic converter. When the catalytic converter reaches operating temperatures, the NOx molecules are thermally desorbed from the sorbent material and delivered to the catalytic converter for effective reduction to molecular nitrogen.
EP 1027919 discloses a system for treating exhaust gases generated from a diesel engine by locating two catalyst components in the engine exhaust gas passage. The first catalyst component which is exposed to oxidising diesel exhaust is located nearest to the engine and is a nitrogen oxide absorbent made of support material carrying precious metal that absorbs nitrogen oxides at low temperature and desorbs them as the temperature is raised during engine operation. The nitrogen oxide absorbent material comprises (a) porous support material selected from the group consisting of alumina, zeolite, zirconia, titania, lanthana and mixtures of any of them and (b) at least 0.1 wt % precious metal selected from the group consisting of platinum, palladium and rhodium or a mixture of any of them based on the weight of a support for the precious metal: platinum carried on alumina is exemplified. The second component is a catalyst such as a lean-NOx catalyst or a selective reduction catalyst which is capable of converting the exhaust gas passing over it including reducing the nitrogen oxides desorbed from the first component into nitrogen (N2) or nitrous oxide (N2O). Materials such as hydrocarbons or ammonia or urea may be injected into the vicinity of the second catalyst component to aid in the reduction.
EP 1203611 discloses an exhaust gas treatment unit for the selective catalytic reduction of nitrogen oxides under lean exhaust gas conditions which contains at least one catalyst with catalytically active components for selective catalytic reduction (SCR components). The exhaust gas treatment unit is characterised in that the catalyst also contains, in addition to SCR components, at least one storage component for nitrogen oxides (NOx components). The NOx storage components preferably contain at least one compound of elements selected from the group consisting of alkali metals, alkaline earth metals and cerium in combination with a nitrogen monoxide oxidation catalyst from at least one of platinum, palladium, rhodium and iridium. Alternatively, or in addition, the catalyst may contain catalytically active components based on support oxides from the group aluminium oxide, silicon dioxide, cerium oxide, zirconium oxide, titanium oxide or mixed oxides thereof catalysed with at least one of the platinum group metals platinum, palladium, rhodium and iridium. Platinum on active alumina is identified as a preferred oxidation catalyst.
WO 00/29726 discloses an apparatus for treating exhaust gas streams, including diesel engine exhaust, which apparatus in one embodiment comprises a catalyst comprising a cerium component and optionally a platinum group metal carried on a flow through monolith substrate followed by a catalysed filter comprising a platinum group metal, a first cerium component and preferably a zirconium component.
WO 2004/025096 discloses a compression ignition engine wherein substantially all fuel for combustion is injected into a combustion chamber prior to the start of combustion, which engine comprising an exhaust system comprising a supported palladium (Pd) catalyst. The supported Pd catalyst may comprise at least one base metal promoter, such as a reducible oxide or a basic metal. The reducible oxide may be an oxide of manganese, iron, cobalt, copper, tin or cerium. The base metal may be an alkali metal, an alkaline earth metal or a lanthanide metal. The catalyst may also comprise a supported platinum component. In an illustrative embodiment, the supported Pd catalyst is palladium supported on ceria (CeO2). A catalyst comprising a physical mixture of Pd/CeO2 and Pt/alumina-based support is also disclosed.
WO 2004/025093 discloses a compression ignition engine operable in a first, normal running mode and a second mode producing exhaust gas comprising an increased level of carbon monoxide relative to the first mode and means when in use to switch engine operation between the two modes, which engine comprising an exhaust system comprising a supported palladium (Pd) catalyst associated with at least one base metal promoter and an optionally supported platinum (Pt) catalyst associated with and/or downstream of the Pd catalyst. The base metal promoter can be any of those disclosed for WO2004/025096 mentioned hereinabove. In one embodiment, the exhaust system comprises a catalyst for catalysing the selective catalytic reduction (SCR) of NOx with at least one NOx-specific reactant disposed downstream of the supported Pd catalyst. Switching between first and second mode running, thereby to promote an exotherm for heating the SCR catalyst downstream, can be done in order to maintain the SCR catalyst at around its optimum temperature range for NOx reduction. NOx specific reactants as described in WO2004/025096 include nitrogenous compounds, for example nitrogen hydride, ammonia, and ammonia precursor e.g. urea, ammonium carbamate and hydrazine.
The SCR catalyst of WO 2004/025096 can comprise the Pt catalyst. Alternatively, the SCR catalyst can be vanadium-based e.g. V2O5/TiO2; or a zeolite e.g. ZSM-5, mordenite, gamma-zeolite or beta-zeolite. The zeolite can comprise at least one metal selected from the group consisting of Cu, Ce, Fe and Pt, which metal can be ion-exchanged or impregnated on the zeolite. In one embodiment, the means for switching between the two modes switches between the first mode and the second mode when the Pt catalyst is <250° C., e.g. less than 200° C. or less than 150° C.
WO 2004/076829 discloses an exhaust-gas purification system for the selective catalytic reduction of nitrogen oxides. The system includes at least one catalyst having catalytically active components for selective catalytic reduction (SCR components). A NOx storage catalyst is arranged upstream of the SCR catalyst in the exhaust-gas purification system. For performing the selective catalytic reduction, metering means for supplying a compound decomposable into ammonia is provided between the NOx storage catalyst and the SCR catalyst. At low exhaust-gas temperatures, the NOx storage catalyst adsorbs the nitrogen oxides contained in the exhaust gas and desorbs them only at rising exhaust-gas temperatures, so that they can be converted by the SCR catalyst when it is active. The NOx storage catalyst includes at least one alkaline compound of elements selected from the group consisting of alkali metals, alkaline-earth metals and rare earths which are coated or activated with at least one of the platinum group metals platinum, palladium, rhodium and iridium. The oxidation activity of the catalyst for nitrogen monoxide may be increased further if the NOx storage catalyst additionally includes catalytically active components based on support oxides selected from the group consisting of aluminium oxide, silicon dioxide, cerium oxide, zirconium oxide, titanium oxide and mixed oxides thereof which are coated with at least one of the platinum group metals platinum, palladium, rhodium and iridium. A particularly preferred NOx storage catalyst includes a storage component based on cerium oxide coated with platinum and additionally platinum as an oxidizing catalyst on a support based on aluminium oxide.
SAE 2000-01-1847 reports the results of using Pt/CeO2 and Pt/SnO2 for treating HCCI engine exhaust gas.
H. Cordatos et al., Journal of Catalysis 159 (1996) 112-118 report that, in idealised laboratory conditions, NO is adsorbed by ceria-supported palladium and that the vast majority of the adsorbed NO is reduced to N2 in a vacuum with increasing temperature (temperature programmed desorption): minimal NO desorption and N2O generation is observed.